1. Field of the Invention
The present invention relates to a ring oscillator operating in a wide output frequency, and more particularly, to a ring oscillator having a wide oscillation frequency range and a simple structure due to additional switches and varactors.
2. Discussion of Related Art
In general, voltage control oscillators (VCOs) are used to adjust oscillation frequencies by voltage control, which are necessary in up-conversion or down-conversion of predetermined frequencies.
The VCOs may be generally classified into ring oscillators, LC oscillators, and crystal oscillators. Among the VCOs, the ring oscillators are widely employed in clock/data recovery circuits and frequency synthesizers because of their simple structures.
FIGS. 1A and 1B illustrate conventional ring oscillators 100 and 100′.
Referring to FIG. 1A, the ring oscillator 100 has a structure in which a plurality of delay cells 100a to 100c form a loop to generate an oscillation frequency. The delay cells 100a to 100c are composed of a series of inverters for inverting and delaying an input signal.
The oscillation frequency of the ring oscillator 100 with such a configuration is variable by delay times of the delay cells 100a to 100c, as follows.
Assuming that N delay cells are arranged in the ring oscillator and the delay time of each delay cell is tD, the total delay time (2×N×tD) counted up by the loop is an oscillation period T.
Therefore, the oscillation frequency fOSC can be given by Equation 1.
                              f          OSC                =                  1                      2            ·            N            ·                          t              D                                                          <                  Equation          ⁢                                          ⁢          1                >            
As can be seen from Equation 1, the shorter the delay time tD of the delay cell used in the ring oscillator, the higher the oscillation frequency becomes. The longer the delay time tD, the lower the oscillation frequency becomes.
In controlling a delay time of the delay cell, there has been a method of varying transconductance by adjusting a bias current supplied into the delay cell.
However, such a bias-current control method has a disadvantage that phase noise of the delay cell abruptly increases when the delay cell is supplied with a small bias current.
Thus, as shown in FIG. 1B, for the purpose of extending an oscillation frequency range, a ring oscillator for high speed employs a plurality of switched capacitors 110 coupled to an output node of the delay cell.
However, the ring oscillator 100′ shown in FIG. 1B is disadvantageous in circuit complexity and chip area due to the arrangement of the switched capacitors 110.
Additionally, the plurality of the switched capacitors 110 may cause a large amplitude fluctuation on the oscillation frequency and thus enlarge a fluctuation of phase noise.